O AND H STABLE-ISOTOPE COMPOSITIONS OF OCEANIC-CRUST AND UPPER-MANTLEROCKS EXPOSED IN THE HESS-DEEP NEAR THE GALAPAGOS TRIPLE JUNCTION

A large variety of rocks, consisting of basalts, dolerites, gabbros an d ultramafics, are exposed on the Intra-rift ridge in the Hess Deep, r epresenting a section of the oceanic crust formed at the East Pacific Ridge [1]. We conducted a stable isotope study of O and H to assess th e nature of the interaction between seawater and the oceanic crust in this region. Fresh basalt and dolerite samples have magmatic MORB valu es (delta(18)O approximate to 6.0, -105 delta D less than or equal to -70) and low H2O+ content (< 0.3 wt%). With one exception, the metabas alts are enriched in O-18 (delta(18)O approximate to 7) and have delta D between -72 and -30 and H2O+ contents higher than 2 wt%. The metado lerites are depleted in O-18 (delta(18)O approximate to 4.9), with hig h delta D (> -57) and H2O+ contents (> 1.3%). Most gabbros have delta( 18)O approximate to 5.6, essentially identical to their initial magmat ic values. Their delta D values are generally higher than -57, suggest ing that they have reacted with some seawater-derived fluids, as is al so suggested by their mineralogy. In gabbros, displaying little or no visual evidence of alteration of their plagioclases and pyroxenes, Del ta(18)O(plagioclase-pyroxene) is small (approximate to 0.3) and consis tent with magmatic values. The calcic (an > 50) plagioclases have magm atic delta(18)O (approximate to 5.8). Albitized, prehnitized plagiocla ses have variable delta(18)O values (4-8). It also appears that the O- 18-modified gabbros are those in which plagioclases are transformed in to albite and/or prehnite, which are characteristic of greenschist fac ies conditions. The lack of calcic (an > 50) plagioclases depleted in O-18 and homblende suggests that most of the gabbros did not react ext ensively under amphibolite facies conditions with seawater-derived flu ids (T greater than or equal to 400 degrees C) or that they were subse quently altered to greenschist facies gabbros. The serpentinized perid otites were depleted in O-18 (3.3-4.9). In one sample, the Delta(18)O( serpentine-magnetite) (approximate to 5-7) corresponds to a temperatur e of about 325 +/- 50 degrees C. These values are compatible with serp entinization by seawater-derived fluids at temperatures between 200 de grees and 350 degrees C. Another sample has a higher delta(18)O value of 10, corresponding to a serpentinization temperature of less than 50 degrees C. The O-18 mass balance between seawater and the oceanic cru st seems to have attained steady-state where enrichment in O-18 Of met abasalts is balanced by the depletion in O-18 of metadolerites. The co ntribution of the gabbros is minor. The depletion in O-18 of the serpe ntinized peridotites is not taken into account since we believe that t he high-temperature serpentinization is characteristic of the Hess Dee p and cannot be extrapolated to the normal oceanic crust.